The calibration of the various transfer functions of mass air flow sensors of the type taught by Sato et al in U.S. Pat. No. 4,393,697 is conventionally conducted on separate calibration test stands. The mass air flow sensor has a sensing head supporting sensing elements which are sensitive to a mass air flow rate. In current automotive applications, the sensing elements are normally entrained in an isolated or bypass channel of the engine air intake system and measures the mass air flow rate of a portion of the total mass air flow. The mass air flow sensor generates an output signal corresponding to the mass air flow rate of the air in the isolated or bypass channel. The relationship of the value of the sensor's output signal to the mass air flow rate is called a transfer function. This transfer function is normally calibrated in three separate steps. First, the sensor is calibrated to produce a specified signal in response to a preselected mass air flow rate. Next, the gain of the sensor is calibrated to generate a specified change in its output signal in response to a preselected change in the mass air flow. Finally, the response time of the sensor's output signal is calibrated to have a predetermined response time in response to a step change in the mass air flow rate.
The calibration of these parameters, in production, are performed at separate test stations while the sensor is mounted on a test stand in which the sensing elements are exposed to mass air flow rates controlled by sonic nozzles. The sonic nozzles accurately control the mass air flow rate required for the desired calibration. The calibration test stands have inlet geometries which condition the inlet air in an attempt to reduce turbulence of the mass air flow being sensed. However, turbulence is almost always inherent in a system where air is drawn in by a pressure drop or vacuum system where it is very difficult to achieve ideal inlet geometry. The generation of turbulence through the air inlet of these calibration stands result in unrepeatable flow fields and an increased signal-to-noise ratio in the output signal generated by the mass air flow sensor.